The present invention relates generally to pyridine compounds, and more particularly to a unique process for preparing 2-chloro-5-substituted pyridine compounds.
As further background, substituted pyridine compounds such as 2-halo-5-substituted pyridines are useful inter alia as insecticides or herbicides, or as intermediates for preparing insecticides, herbicides and pharmaceuticals. For example, one such compound, 2-chloro-5-methylpyridine, finds use as an intermediate to imidacloprid, a compound receiving substantial attention as an insecticide.
As to their preparation, 2-halo-5-substituted pyridines can be prepared by halogenation of a corresponding 5-substituted pyridine. However, such halogenations are difficult to control and generally not selective, leading to mono- and dichlorination on alkyl substituents at the 5 position and substantial dichlorination of the pyridine ring at the 1 and 6 positions.
2-Halo-5-substituted pyridines can also be prepared from their corresponding 2-amino-5-substituted pyridines as, for example, described in German Patent No. 4111214 to Bayer A. G. However, this route requires the formation of the aminopyridine and would be relatively disadvantageous compared to an effective process directly providing the desired 2-halo-5-substituted pyridine.
U.S. Pat. No. 5,099,025 describes the preparation of 2-chloro-5-methylpyridine, which involves reacting the N-oxide of 3-methylpyridine with certain sulphonyl or sulphanoyl chlorides in the presence of a basic organic nitrogen compound and a diluent at a temperature between about -120.degree. C. and 150.degree. C. However, such processes require the formation of the N-oxide and are also of a relatively complex nature.
Additional background information to the present invention may be found in U.S. Pat. Nos. 5,053,516; 4,504,664; 5,229,159; and in European Patent Applications 108483 (1984); 121320 (1984) and 546418 (1993).
In light of the background in the area, there remains a need for a relatively simple process for preparing 2-halo-5-substituted pyridine compounds such as 2-chloro-5-methylpyridine. Preferred processes would minimize the number of steps required to achieve the desired product and utilize readily available and inexpensive starting materials. The applicant's invention addresses these needs.